As wireless communication systems are making the transition from wireless telephony to interactive internet data and multi-media types of applications, the desire for higher data rate transmission is increasing tremendously. As more and more devices go wireless, future technologies will face spectral crowding and coexistence of wireless devices will be a major issue. Considering the limited bandwidth availability, accommodating the demand for higher capacity and data rates is a challenging task, requiring innovative technologies that can coexist with devices operating at various frequency bands. Ultrawideband (UWB), which has been considered primarily for use with radar applications in the past, offers attractive solutions for many wireless communication areas, including wireless personal area networks (WPANs), wireless telemetry and telemedecine, and wireless sensors networks. With its wide bandwidth, UWB offers a capacity much higher than the current narrowband systems. While conventional narrowband communication systems employ radio frequency (RF) carriers of much higher frequency than the information rate required for transmitting baseband signals, UWB is a carrierless (baseband) transmission system. As such, UWB does not require the necessary up/down conversion of conventional communication systems, resulting in a much simpler solution and less expensive circuitry. Other benefits of UWB include immunity to multi-path effects, high resolution (sub-decimeter range), robustness against eavesdropping, and easier material penetration.
Simple, power efficient, low cost, and small sized UWB transceiver design is a challenging task. There are several receivers proposed for UWB communication. Fully coherent receivers, like rake receivers, and correlator receivers perform well but at the expense of extremely high computational and hardware complexity. In general, a coherent receiver requires several parameters regarding the received signal and radio channel. The multipath delays, the channel coefficients for each delayed multipath component and the distortion of the pulse shape need to be estimated for optimal coherent reception. Note that in UWB, the number of multipath components is very large and it is not uncommon for UWB to comprise hundreds of multipath components. Note also that given the total constant transmitted power, the power in each of these multipath components will be very low. Therefore, estimating the delays and coefficients from the received multipath components is an extremely challenging task. Therefore, receivers that relax these estimation requirements would be preferable.
Non-coherent receiver designs in UWB relax the amount of information that needs to be estimated accurately for the detection of the transmitted bits. In other words, the synchronization, channel estimation, and pulse shape estimation is not as stringent as in the case of the fully coherent receivers. Some of the non-coherent transceiver designs known in the art include a transmitted reference (TR) based UWB transceivers, an energy detector, and a differential detector. Common to all these non-coherent transceiver designs is that the channel estimation and received pulse estimation are not necessary. Also, the timing estimation is easier and the receiver performance is more immune to the timing mismatch.
FIG. 1 illustrates a coherent rake receiver 10 that is well known in the art. With a coherent rake receiver, the received signal energy over several multipath components 20 are captured with the correlators 15 at each finger and these fingers, or correlator outputs, are combined 25 to make a bit decision 30. The correlator 15 at each finger requires the templates 35 for the received pulse to match to the received signal's pulse. The correlators 15 in each finger need to be synchronized to the exact received multipath position. In the case of a coherent rake receiver 10 the optimal combining requires the channel coefficient and/or signal-to-noise-ratio estimates for efficient combining.
FIG. 2 illustrates three different non-coherent receivers currently known in the art. FIG. 2(a) illustrates TR-based receiver. FIG. 2(b) illustrates an energy detector and FIG. 2(c) illustrates a differential detector. In all these types, the integration time 40 is larger than the duration of the pulse. Often the integration time is on the order of the maximum excess delay of the channel. Note that none of these receivers require a local template to correlate the received signal. The TR-based receiver and differential detector correlate the signal with a delayed version of itself 45. In a TR-based receiver the delayed version of the signal is the known reference signal. In the differential detector, both of the signals are data, but, the decision is made on the difference of the correlated signal. Note that in non-coherent receivers the analog received signal is sampled after the integrator, often utilizing integrate-dump circuitry.
While the integration time in a non-coherent receiver should be large enough to accumulate all the signal energy, the integration should not be too large as to receive excess noise. The optimal integration time depends upon the maximum excess delay of the channel and the noise power. Accordingly, an efficient, accurate and low complex delay spread and/or maximum excess delay estimation is needed in the art to provide a non-coherent receiver that maximizes the data rate for a user while also reducing the inter-pulse interference of the signal.